The present invention relates to a porous membrane having a single layer structure and a method of preparing the same. More particularly, it relates to a porous membrane capable of losing its permeability at a temperature equal to or higher than a prescribed temperature. The present invention also relates to a battery separator formed of the porous membrane and a battery equipped with the same.
Porous polymer membranes have been used in the field of filtration and separation technology. Various techniques have been used to impart certain desired characteristic features to such polymer membranes. For example, the membrane may be stretched in order to improve its strength. When it is to be used in filtration, the membrane may be treated with a surfactant to improve its affinity to filtering solutions. Ion exchange function have been imparted to membranes by grafting or copolymerizing various monomers having specific functional groups.
In certain applications, it is desired to have a porous polymer membrane which is capable of losing permeability above a prescribed temperature. For example, such a membrane would be highly desired as the separator component in rechargeable lithium secondary batteries. The membrane must have a property of permitting free passage of ions contained in the battery's electrolyte via passage through the membrane's porosity ("ion permeability" or "electrolytic conductivity") under normal operating conditions. However, such permeability must be sufficiently reduced to cause the electric current to shut down when the temperature within the battery rises beyond a certain point due to malfunctioning during the charging process or to short circuiting between the electrodes or for other reasons. If such electric current shut down is not achieved, the vapor of the solvent used in the electrolyte solution may cause excessive increase in pressure within the battery to create a danger, such as fire or explosion.
In the past, kraft paper or Manila hemp sheet material was used as a separator employed in conventional batteries, lithium batteries and capacitors. More recently, non-woven fabrics and porous polyolefin membranes having a high mechanical strength have been used for this purpose. However, these membranes do not exhibit the ability to reduce electrolytic conductivity to shut down the battery's current.
In particular, lithium batteries are designed to allow the flow of a high density current, and accordingly, the temperature within the battery will rapidly rise when short circuiting occurs between the electrodes. The short circuit causes an overcurrent which, in turn, accelerate chemical reaction at the anode. Rapid increase in temperature within the battery is extremely dangerous. If there is no protective mechanism, temperatures of 140.degree. C. and greater may be quickly reached. This condition may cause the organic solvent used in the electrolyte solution to ignite or may even cause explosion of the battery. To avoid such dangers, a variety of countermeasures are presently needed. Because of the need for commercial products to be safe, there is an increased market demand to have a mechanism which can protect against such dangers.
One of the proposals to solve such a problem is to use, as a battery separator, a material which fuses and, thereby, becomes nonporous above its melting point. Under exothermic conditions, the separator would lose its ion permeability, resulting in preventing the flow of overcurrent. Polyolefins have been proposed as a material for such purposes. However, the mechanical strength of polyolefins decreases with decreased melting points. Thus, materials having sufficient strength to be suitable as the separator material are, for example, polypropylenes or high density polyethylenes which have melting points of 130.degree. C. or higher. However, with such materials, the porosity merely commences to lessen after the battery temperature has reached 130.degree. C. or higher and, therefore, the risk of fire or explosion caused by abnormal heating is high. Such separators are unsuitable to achieve the desired result.
Recently, a battery separator of a multilayer structure has been proposed as a means of shutting down the battery circuit under exothermic conditions. Such separator comprises a porous polymer membrane having a high melting point as the support layer and a porous polymer membrane having a low melting point as the fusible layer. The membranes are laminated so that the porous polymer membrane having a low melting point can fuse at a temperatures above its melting point to form a non-permeable layer while providing the strength of the higher melting point membrane. For example, a battery separator made of a multilayered porous polyolefin membranes is disclosed in Japanese Patent Publication (Kokai) SHO 62-10857(1987), by laminating a non-crosslinked polyethylene film on a crosslinked polyethylene film is disclosed in Japanese Patent Publication (Kokai) HEI 3-59947(1991) and of a non-woven fabric having a coating of wax thereon is disclosed in U.S. Pat. No. 4,741,979. Also, a lithium battery separator having a multilayer structure which comprises a support layer of a porous or a finely porous polymer and a fusible unwoven fabric thermally adhered to the support layer is disclosed in Japanese Patent Publication (Kokai) HEI 2-75152(1990). For reference, a schematic view of the cross section of a porous membrane having a multilayer structure formed by laminating a fusible porous polymer layer on a porous polymer layer as the support layer is illustrated in FIG. 2.
However, battery separators formed from multilayered porous polymer membranes have problems of thickness and uniformity. In small lithium batteries, the thickness of the membrane as the battery separator has to be reduced and, at the same time, made uniform in order to provide increased energy density per unit weight or volume. Stated another way, the present state of the art requires the use of porous membranes formed from polymers with high melting points (first order transition temperatures) to achieve the needed mechanical strength of the membranes in order for it to maintain its functionality as a separator. However, because it is necessary for the electrolytic conductivity to be completely blocked when the battery temperatures rise to about 130.degree. C. it is necessary to use a material having low melting points in order to completely block the membrane pores. It is clear that the needs of high mechanical strength and of the capacity to become substantially non-porous by fusing or the like are contradictory according to present polymer technology.
Although the multilayered separator provides both strength and capacity to become substantially non-porous at prescribed temperatures, such separators have a greater thickness than desired to achieve high energy density, and may form a skin like structure of low porosity between the laminated layers at the time of lamination of the different layers of the membrane. As a result, the permeability of the multilayered membrane may be substantially reduced, lost or blocked by such a skin layer.
It is highly desired to have a single layer porous membrane which is thin, has high mechanical strength and is capable of becoming substantially non-porous at low temperatures (e.g. 80.degree.-130.degree. C.) without loss of mechanical strength. Such a membrane would be suitable as a separator in a battery to provide a safety means against overheating and catastrophic results therefrom